In recent magnetic disk drives, in accordance with increases in magnetic recording density, there is a tendency for the circumferential length of a mark recorded on a magnetic recording medium (namely, a magnetic disk) to become shorter, and for the radial width of a track on the magnetic disk to become narrower. Accordingly, to enhance the quality of read/write signals, it is necessary to narrow the distance (more specifically, magnetic spacing) between the magnetic disk and a magnetic head.
In light of the above, in the recent magnetic disk drives, the magnetic head has a heating element for adjusting the distance between the magnetic head and the magnetic disk (more specifically, the distance between the read/write element of the magnetic head and the magnetic disk) by thermal expansion of the magnetic head. In other words, in the recent magnetic disk drives, the dynamic flying height of the magnetic head flying over the magnetic disk can be adjusted.
The dynamic flying height of the magnetic head can be easily controlled by power called dynamic flying height power (hereinafter referred to as “the DFH power”) supplied to the heating element. However, unless the state (touchdown state) in which the magnetic head touches the magnetic disk is detected, it is difficult to execute accurate flying height control.
In the prior art, the touchdown (TD) of the magnetic head is detected based on, for example, a track position error signal (PES), as follows: Firstly, when tracking control is executed with the magnetic head kept flying normally, the PES has a preset value. A change in the PES indicates the accuracy of positioning. When the flying height of the magnetic head is reduced to cause the magnetic head to touch the magnetic disk, vibration due to the touch occurs in the magnetic head radially with respect to the magnetic disk. At this time, an abnormal change in the PES can be observed. Utilizing this phenomenon, the touchdown of the magnetic head can be detected.
Further, when the magnetic head touches the magnetic disk, a vibration component is observed along the normal line of the magnetic disk. This state appears as a variation in the amplitude of a read signal. In the case of, for example, a servo signal or a data signal, a change in the read signal can be detected as the value of a variable gain amplifier (VGA) incorporated in a read channel (RDC).
The phenomenon in which the magnetic head vibrates due to touchdown generally depends upon the radial position on the magnetic disk. At the inner and outer circumferential portions of the magnetic disk, vibration of a greater magnitude appears in the magnetic head due to touchdown, whereas at the (radially) middle portion of the disk, vibration of a smaller magnitude occurs in the magnetic head. This is because at the middle portion, the contact friction force of the magnetic head is influenced by a skew angle to thereby produce a vibration force exerted therein radially with respect to the disk. As a result, it is easy to detect touchdown at the inner and outer circumferential portions of the disk, while it is difficult to detect the same at the middle portion since the signal level is low. Furthermore, measured values for detecting touchdown vary because of the influence of warpage of the magnetic disk or because of the lubricant agent on the disk.